I used red lead in my loco tenders for 25 years without one problem but it is getting very hard to get and my last loco tender I used industrial red oxide paint and so far has been ok (now 5 years old )I would pour the paint in and rotate the tender slowly and let the paint get into all the corners then pour out the excess and allow to dry for up to 7 days mark

You need to use a product called Apexior 3, it is made especially for inside of water tanks. Isoflex is not designed to be submerged in water for long periods. You can obtain Apexior products from heritage steam